Deprecate (and fix) CartesianPoints

This addresses an oversight from the APE5 coordinate work that @mhvk pointed out in #2712: `CartesianPoints` should have been deprecated in favor of `CartesianRepresentation`.  It can't be removed outright because it was in the 0.3 API, but its role is entirely subsumed by `CartesianRepresentation` (although the API is a bit different).

Incidentally, it's also broken in 0.4, so this also makes some changes to make it work.

It's possible no one out there actually uses `CartesianPoints`, but it's safer to do this just in case, I think.  We'll plan on removing it completely in 1.0.

@embray or @astrofrog, do you object to this going into 0.4 ?  It is technically a bug (in that `CartesianPoints` is broken in the RC), but a bug in a minor (possibly unused) feature.  More important is getting the depreaction warning in now so we can remove in 1.0.